Fluidic angular rate sensors of the jet deflection type are well known in the art. One which has found wide acceptance is the hot-wire anemometer type disclosed in U.S. Pat. Nos. 3,626,765 to Moore et al and 4,020,700, of common assignee herewith, to Lopiccolo et al. In each, balanced bridge temperature sensitive elements disposed at one end of a jet chamber housed within a fluid filled casing, are cooled differentially by a fluid jet in dependence on the Coriolis deflection of the jet during angular rotation of the sensor. In the presence of differential cooling a resultant bridge imbalance produces a differential signal whose magnitude is proportional to the angular velocity of the sensor. The absence of rotation, i.e. the null state, results in equal cooling of the elements and the differential output signal from the bridge is ideally zero.
The sensitivity (volts/degrees/second) and frequency response of the sensor are both dependent on the flow rate of the fluid jet. In both the Moore and Lopiccolo et al sensors the fluid jet is introduced into the jet chamber by a nozzle which receives the fluid under pressure from a jet pump assembly of the type described in U.S. Pat. No. 3,587,328 to Schuemann. This pump assembly includes a diaphragm comprising two piezoelectric material discs coated with a conductive metal film and bonded face to face to a central conductive plate with conductive epoxy. The diaphragm in turn is bonded along the periphery of one disc to one end of a cylindrical, centrally apertured flexible supporting element e.g. flexure, which is clamped at the other end within the sensor casing by a closure plate. The closure plate is epoxy bonded in place within the casing, and forms the end wall of the pump chamber formed by the internal diameter of the cylindrical flexure and the bonded diaphragm.
The piezoelectric discs are polarized to make the electrical axis (X-axis) of the disc perpendicular to the main plane (Y-Z axes), and the discs are mounted to the central plate such that their X-axes are oppositely poled. The diaphragm is electrically connected through the conductive coating into the regenerative loop of an oscillator. The applied AC voltage develops a bending moment on the diaphragm which causes the two discs to expand and contract oppositely on each alternating half cycle of the excitation signal. As a result the diaphragm oscillates, and in each cycle forces fluid from the pump chamber through a pump orifice along the sensor plenum to the jet nozzle, all of which is shown and described by Schuemann.
In the sensor configuration disclosed in Lopiccolo et al the impulse jet pump assembly is essentially identical, with the exception that the pump closure plate, termed an anvil in the '700 patent, is secured within the sensor housing by force of a conical spring held in compression against the anvil with a threaded annular lock nut which is tightened against the spring. This is in contrast to the Schuemann assembly where the closure plate (anvil) is secured within the sensor casing with epoxy. In either case, the anvil or closure plate clamps the support flexure of the pump in place by the force exerted against the support rim of the flexure.
In either pump assembly the diaphragm ideally oscillates at its natural frequency, and the frequency together with the amplitude of displacement determine the cubic feet per minute (CFM) flow rate of the fluid jet in the chamber. The operation of the pump at its natural frequency is obviously desirable, due both to the stability of the frequency of oscillation and to the lower power requirements for maintaining oscillation. The stability of oscillation is most important from the standpoint of maintaining the calibration accuracy, i.e. null offset value, of the sensor. The pump natural frequency value is dependent on a number of factors including the geometry of the diaphragm, the pump chamber, and the pump orifice, and also on the viscosity of the inert gas fluid. The displacement amplitude of the diaphragm (and the amplitude of the fluid pressure pulsations at the output of the pump orifice) is dependent on the damping coefficient of the pump assembly which has a value determined by the aggregate contribution of the various elements of the pump, including the manner in which they are joined to provide the complete assembly.
The largest single contributor in determining both the frequency of oscillation and the displacement amplitude is the diaphragm assembly itself. In the sensor assemblies of Moore et al and Lopiccolo et al, both of which use the basic pump assembly disclosed by Schuemann, the diaphragm is mounted to the flexure along a circumferential band at the periphery of one of the piezoelectric discs. The width of the band is determined essentially by the width of a mounting surface rim on the flexure. This type of mounting results in two distinct disadvantages, or limitations in the Schuemann pump, namely: (1) the maximum mechanical strength of the diaphragm/flexure bond is limited to the peel strength of the metal coating on the piezoelectric disc, and (2) the bond suspends the diaphragm within a plane which is offset from the plane which includes the natural bending point of the diaphragm. While the peel strength of the coating may be sufficient to support the actual mounting of the diaphragm to the flexure, any weakening of the coating may produce a creep or hysteresis in the diaphragm to flexure bond, resulting in a corresponding shift in the damping coefficient and frequency of oscillation. Similarly the mounting of the diaphragm along the surface of the pump chamber side disc produces a nonsymmetry in that it offsets the diaphragm from its natural bending spot, or axis of deformation, which lies in a central plane between the two piezoelectric discs. This produces a nonsymmetry in the radial expansion and contraction of the diaphragm during oscillation, which may result in an offset of the frequency of oscillation from the natural frequency value.
Any offset errors arising out of the diaphragm mounting may be accounted for in the initial calibration of the sensor. In other words they may be calibrated out during manufacture. The major problem caused by the mounting arises over the life of the sensor, i.e. with the long term aging effects. The major attraction of this type of fluidic angular rate sensor lies in the fact that there are no rotating parts, and other than the oscillation of the diaphragm, there are no other moving parts within the sensor. As such the sensor has utility in long storage, or long shelf life applications, which require that the sensor maintain calibration accuracy for long time intervals. Some as long as ten years. The aging and hysteresis effects on the actual bond strength, and the unpredictability of the diaphragm oscillation due to the non-symmetric mounting, tend to reduce the sensor ability to maintain long term calibration accuracy.
Another problem associated with long storage life applications is the possibility of degree of depolarization of the piezoelectric diaphragm during storage. This, when the sensor is later actuated, reduces the displacement amplitude of the diaphragm during oscillation which in turn affects the amplitude of the pressure pulsations and the CFM flow rate of the fluid jet. It would be desirable to repolarize the piezoelectric wafers of the diaphragm prior to sensor actuation following the storage time interval, in a manner similar to the automatic recalibration routine provided for in U.S. Pat. No. 4,026,159 to Isakson et al where the sensor bridge is automatically renulled immediately prior to actuation of the pump assembly. The prior art assemblies do not have the ability to provide such a repolarization due to electrical inaccessibility to the internal surfaces of the piezoelectric discs.